Techniques for producing hydrogen or a synthetic gas constituted of hydrogen, carbon monoxide and other compounds from a natural gas containing methane as its main ingredient or another hydrocarbon gas include steam reforming, partial oxidation reforming and carbon dioxide gas reforming. Table 1 lists the formulae representing such reforming reactions and chemical reactions that may occur during these reforming reactions as well as related thermodynamic constants, taking methane as an example starting material.
TABLE 1TΔGΔHTΔSReaction(K)(kJ/mol)(kJ/mol)(kJ/mol)(1)SteamCH4 + H2O → CO + 3H21000−27.0225.7252.7reforming(2)Partial oxidationCH4 + ½ O2 → CO + 2H21000−219.6−22.1197.5reforming(3)Carbon dioxideCH4 + CO2 → 2CO + 2H21000−23.8260.5284.3reforming(4)Shift reactionCO + H2O → CO2 + H21000−3.1−34.8−31.7(5)CompleteCH4 + 2O2 → CO2 + 2H2O1000−800.7−800.50.2oxidation reforming
In practical settings, these basic reaction formulae are combined with each other, and the reactions used to produce hydrogen are, for example, described as follows:(1)+(4)CH4+2H2O→CO2+4H2ΔH=190.9 kJ/mol (endothermic)  Formula (6)(2)+(4)CH4+1/2O2+H2O→CO2+3H2ΔH=−56.9 kJ/mol (exothermic)  Formula (7)((1)+(4)+(5))/2CH4+O2→CO2+2H2ΔH=−304.8 kJ/mol (exothermic)  Formula (8).
Starting from the top, these reactions are called steam reforming, autothermal reforming and partial oxidation reforming. Production of a synthetic gas also follows a similar manner, and it is based on the following formula:((1)+(2))/2CH4+1/4O2+1/2H2O→CO+5/2H2 (endothermic)  Formula (9).
This formula represents commonly-used autothermal reforming in which steam and oxygen are used in combination.
Considering the features of such reactions in terms of the substance (oxygen or steam) used to reform the gas, the use of steam is excellent in the production quantity of hydrogen but has difficulty in starting the reaction because of its high endothermic tendency. On the other hand, the use of oxygen is not so excellent in the production quantity of hydrogen but allows the reaction to be readily started because of its high exothermic tendency. Here it should be noted that the steam reforming (Formula (1)) requires heat for the combustion of one mole of hydrogen molecules to produce hydrogen molecules at the quantity more by one mole than those the partial oxidation reforming (Formula (2)) does. Therefore, under the conditions without exhaust heat having a temperature high enough to allow the steam reforming, both the steam reforming and the partial oxidation reforming would achieve the very same theoretical efficiency. So if pure oxygen is made easily available at low cost in some way, then the partial oxidation reforming provides an ideal reforming method excellent in both ease in starting the reaction and efficiency.
As a method that can provide the partial oxidation reaction and oxygen isolation simultaneously, partial oxidation reforming using an oxygen-permeable membrane (also called an oxygen-isolating membrane or an oxygen-permeating ceramics) has recently attracted attention. This method uses a material having both oxide ionic conductivity and electronic conductivity (an oxide-ion/electron hybrid conductor) as a separator that partitions a hydrocarbon gas and air, thereby isolating pure oxygen necessary in the partial oxidation reforming from the air by taking ΔG of the partial oxidation reaction (in more specific terms, the oxygen concentration gradient) as a driving force, in order to promote further partial oxidation reactions at the side of the hydrocarbon gas. Patent Documents 1 and 2 are disclosed as patents based on this principle. Many kinds of materials of oxygen-permeable membranes, such as ones described in Patent Documents 3 to 5, have also been reported.
However, such an oxygen-permeable membrane requires ionization of oxygen atoms for permeation thereof, and thus the oxide ion transport and the electron transport in the reverse direction cause the generation of large Joule heat (heat generation due to electric resistance) within the membrane. So in a system with insufficient thermal balance, such a membrane would be lacking in oxygen permeability, and eventually broken. As a countermeasure against this problem, Patent Document 1 discloses a technique in which the heat generated in association with the partial oxidation reforming reaction (ΔH) is regarded as excess heat and is removed by the endothermic reaction of the steam reforming until the most of the heat is spent; however, the necessity of removing Joule heat generated within the oxygen-permeable membrane is not discussed in this patent document.
Furthermore, there has been no disclosure about an effective reforming apparatus that can downsize a partial oxidation reforming system using an oxygen-permeable membrane.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-26103
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2005-145760
Patent Document 3: WO2003/084894
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-93325
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2005-281077
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2005-281086
Patent Document 7: Japanese Unexamined Patent Application Publication No. 2004-149332
Patent Document 8: Japanese Unexamined Patent Application Publication No. 2003-2610